v3d/drm-shim: emulate a rpi4 instead of a rpi3

7278 is the chip on the rpi3, while the rpi4 that made it to market has the 2711 chip.

When this was introduced (82bf1979), the rpi4 was probably still in flux, which is why the rpi3 chip was put there (and v3d doesn't care about that, but v3dv does).